This project will study neural control of locomotor orientation in the developing golden hamster, mesocricetus auratus, in order to determine whether specific structural changes can be demonstrated in particular fiber tracts as they acquire behavioral function. Hamster orientation is controlled largely by thermal stimuli during the first week of life, a period when midbrain but not forebrain lesions cause deficits in orientation. Between the 7th and 10th day of life, thermal control weakens and the pups begin to display rhythmic exploratory behavior with orientation to olfactory stimuli. Some forebrain lesions can delay this behavioral change, suggesting that the shift may reflect the maturation of descending influences from the forebrain. The proposed experiments will (1) quantitatively describe the transition from thermal to olfactory control by measuring the effect of age on orientation to competing thermal and olfactory stimuli of different intensities; (2) use lesions to identify brain tracts subserving thermal and olfactory orientation behavior; (3) use a new application of a standard histological technique to evaluate maturational changes in these fiber tracts before and after the transition to olfactory control. Pups with fiber transections will be sacrificed after three days of behavioral testing, their brains stained with the Fink-Heimer technique, and the maturational stage of the transected tracts assessed by measuring the size of the field of long- lasting terminal degeneration. This method of assessing maturation will be used because it has recently been found that the onset of long-lasting degeneration in optic tract terminals in the superior colliculus coincides with the age of eye opening and the beginning of adult-type optic tract synapse formation. This suggests that the onset of long-lasting degeneration may be a sign of important structural and functional changes in the neurone. It is the purpose of the proposed experiments to determine whether behavioral consequences can be shown to accompany this structural maturation change. If successful, this method would have many applications in studies of neural and behavioral development.